For the purpose of environmental load reduction and environmental protection recently needed on a global scale, the cleaning of exhaust gases for reducing the emission of air-polluting materials, and the improvement of fuel efficiency (low fuel consumption) for suppressing the emission of CO2, a cause of global warming, are strongly required in automobiles. To clean exhaust gases, and to improve fuel efficiency in automobiles, various technologies such as the development of engines with high performance and fuel efficiency, the cleaning of exhaust gases, the weight reduction of car bodies, the air resistance reduction of car bodies, efficient power transmission from engines to driven systems with low loss, etc. have been developed and employed.
Technologies for providing engines with high performance and improving their fuel efficiency include the direct injection of fuel, increase in compression ratios, decrease in displacements by turbochargers, the reduction of engine weights and sizes (downsizing), etc., and are used not only in luxury cars but also in popular cars. As a result, fuel combustion tends to occur at higher temperatures and pressure, resulting in higher-temperature exhaust gases discharged from combustion chambers of engines. For example, the temperatures of exhaust gases are 1000° C. or higher even in popular cars, like luxury sport cars, so that the surface temperatures of exhaust members tend to exceed 950° C. Because exhaust members exposed to high-temperature oxidizing gases are subjected to repeated heating/cooling cycles by the start and stop of engines in a severer oxidizing environment than ever, they are required to have higher heat resistance such as oxidation resistance, high-temperature strength, thermal fatigue life, etc. than ever.
Exhaust members such as exhaust manifolds, turbine housings, etc. used for gasoline engines and diesel engines of automobiles have conventionally been formed by castings with high freedom of shape, because of their complicated shapes. In addition, because of their severe, high-temperature use conditions, heat-resistant, cast irons such as high-Si, spheroidal graphite cast irons and Niresist cast irons (Ni—Cr-containing, austenitic cast irons), heat-resistant, cast ferritic steels, heat-resistant, austenitic cast steels, etc. are used.
However, conventional, heat-resistant, cast irons such as high-Si, spheroidal graphite cast irons and Niresist cast irons exhibit low strength and low heat resistance such as oxidation resistance and thermal fatigue life in environment exposed to exhaust gases at higher than 900° C., despite relatively high strength when exhaust gases are at 900° C. or lower, and exhaust members are at about 850° C. or lower. The heat-resistant, cast ferritic steel is usually poor in high-temperature strength at 900° C. or higher.
As a material that can withstand higher temperatures than heat-resistant, cast irons and heat-resistant, cast ferritic steels, there is a heat-resistant, austenitic cast steel. For example, WO 2005/103314 proposes a high-Cr, high-Ni, heat-resistant, austenitic cast steel comprising by weight 0.2-1.0% of C, 3% or less of Si, 2% or less of Mn, 0.5% or less of S, 15-30% of Cr, 6-30% of Ni, 0.5-6% of W and/or Mo (as W+2 Mo), 0.5-5% of Nb, 0.01-0.5% of N, 0.23% or less of Al, and 0.07% or less of O, the balance being substantially Fe and inevitable impurities. Because this heat-resistant, austenitic cast steel has high high-temperature yield strength, oxidation resistance and room-temperature elongation, and an excellent thermal fatigue life particularly when exposed to an exhaust gas at a high temperature of 1000° C. or higher, it is suitable for exhaust members, etc. of automobile engines.
Because cast exhaust members are subjected to machining such as cutting in connecting portions such as surfaces attached to engines and their surrounding parts and mounting holes, portions needing dimensional precision, etc., and then assembled in automobiles, they should have high machinability. However, heat-resistant cast steels used for exhaust members are generally difficult-to-cut materials having poor machinability. Particularly heat-resistant, austenitic cast steels comprising much Cr and Ni for high strength are poor in machinability. Accordingly, when exhaust members made of a heat-resistant, austenitic cast steel are cut, relatively expensive cutting tools having high hardness and strength are needed, and frequent tool exchange is necessary because of a short tool life, resulting in high machining cost, and long cutting time because of a low cutting speed, resulting in low machining efficiency because cutting needs a long period of time. Thus, exhaust members made of the heat-resistant, austenitic cast steel suffer low productivity and economy in machining. From the aspect of machinability, it has been found that the heat-resistant, austenitic cast steel of WO 2005/103314 has room to be improved.